Conventionally, as the sealed linear actuator unit of this type, there is known a linear actuator unit disclosed in JP63-172545 U. The linear actuator unit includes an outer rail, an inner block, and a slide plate. The outer rail is formed into a cylindrical shape so as to include a hollow portion, and includes a slit-opening portion formed along an axial direction thereof. The inner block travels in the hollow portion along a track rail arranged in the outer rail. The slide plate is arranged outside the hollow portion of the outer rail, and is coupled to the inner block through the slit-opening portion. By fixing an object to be conveyed to the slide plate, it is possible to cause such objet to be conveyed to freely move along the outer rail.
Further, in the hollow portion of the outer rail, there is arranged a screw shaft, and the inner block is screwed into the screw shaft. Thus, by rotating the screw shaft by a motor arranged at one end of the outer rail, it is possible to provide the inner block with a moving direction and a moving distance corresponding to a rotational direction and a rotational amount of the screw shaft. Further, it is possible to provide an arbitrary linear reciprocating movement to the slide plate installing thereon the objet to be conveyed.
In addition, in the conventional linear actuator unit, in order to prevent dust from getting into the outer rail through the slit-opening portion, there is provided a belt-like cover covering such slit-opening portion. The cover is tensioned between both ends of the outer rail. Both edge portions along a longitudinal direction of the cover is held in contact with a pair of flange portions of the outer rail, the flange portions being opposed to each other while sandwiching the slit-opening portion. In this way, the cover closes the slit-opening portion.
In order to allow the reciprocating movement of the slide plate in a state in which the slit-opening portion is covered with the cover, roller groups for lifting up the cover from the outer rail are provided on a front side and a rear side of the slide plate, respectively. When the slide plate approaches the cover, the cover is separated from the outer rail by a roller group positioned on the front side of such slide plate. After the cover passes through the slide plate, the cover is, in turn, pressed against the outer rail by a roller group positioned on the rear side of such slide plate.    Patent Document 1: JP 63-172545 U